Seven Marines have been killed in an explosion during a Marine Corps training exercise last night in Nevada.

The mortar blast occurred at the Hawthorne Army Depot in Nevada, which houses and disposes of ammunition.

The Marine Corps said in a statement this morning that the accident, at about 10pm last night, also injured several with the 2nd Marine Division, based in Camp LeJeune, North Carolina.

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Historic: Hawthorne has held an important place in U.S. military history since World War II when it became the staging area for ammunition, bombs and rockets

Demonstration: This file photo provided by the U.S. Marine Corps, shows mortarmen firing off illumination rounds during a training exercise at the Hawthorne Army Depot

Practice facility: The mortar blast occurred at the Hawthorne Army Depot in Nevada, where members of the U.S. military conduct training

The Marines immediately issued an indefinite moratorium on firing of all such mortars worldwide until an investigation clears as safe the type of weapon and ammunition in the tragedy.

A Marine Corps official, speaking on the condition of anonymity, said it was not immediately clear whether the mortar exploded prematurely inside its firing tube or whether more than a single round exploded.

The 60mm mortar is a weapon that traditionally requires three to four Marines to operate, but it's common during training for others to observe nearby.

'We remain focused on ensuring that they are supported through this difficult time. We mourn their loss, and it is with heavy hearts we remember their courage and sacrifice.

The cause of the explosion is under investigation.

Tragedy: Marine Brig Gen Jim Lukeman speaks to the media on Tuesday outside the gates of Camp Lejeune, where the Marines were based

Tribute: A sign showing support for the seven U.S. Marines killed in the explosion is pictured near the site of the accident on Tuesday

Area: The facility is made up of hundreds of buildings spread over more than 230 square miles. It is located about 140 miles away from Reno

Preparedness: The military has called Hawthorne an ideal training facility for special forces

The injured were taken to area hospitals for treatment and further evaluation.

The identities of those killed were not released pending notification of their families and a 24-hour waiting period after that.

Russ Collier told KRNV-TV
in Reno that the explosion was an accident unrelated to the ammo that
is kept at the military facility near the small desert community of
Hawthorne, about 140 miles southeast of Reno.

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Calls to the depot and to U.S. Joint Munitions Command, which oversees the facility, were not immediately answered.

The Mineral County sheriff's office said it does not have any details.

Hawthorne has held an important place in American military history since World War II when it became the staging area for ammunition, bombs and rockets for the war.

The Nevada Division of Environmental Protection says that the depot employed more than 5,500 people at its peak.

The facility is made up of hundreds of buildings spread over more than 230 square miles.

Location: The facility is made up of hundreds of buildings spread over more than 230 square miles in Nevada. It is located about 135 miles from Reno

Nevada was chosen for the location
because of its remoteness in the wake of a devastating explosion at the
government's main depot in New Jersey in the 1920s.

It opened in September 1930 as the
Naval Ammunition Depot Hawthorne, was redesignated Hawthorne Army
Ammunition Plant in 1977 when it moved under the control of the Army,
according to its website.

In 1994, the site ended its production mission and became Hawthorne Army Depot.

The
site currently serves several purposes for the military, including
storing ammunition and explosives and providing what the military calls
an ideal training facility for special forces preparing for deployments
to similar desert terrain in places like Afghanistan.

Nevada's political leaders expressed their sympathy.

Senate Majority Leader Harry Reid gave his condolences to victims of the explosion during a Tuesday morning speech on the Senate floor.

Blast: The cause of the explosion at the depot, pictured, is under investigation, and a moratorium on mortars has been imposed worldwide

He said: 'My thoughts are with those who were injured. My heart goes out to the families of those who lost their lives. And my sympathies are with their fellow Marines, who are also grieving this loss.'

Nevada Republican Sen Dean Heller
tweeted: 'Thoughts and prayers are with the families who lost a loved
one in the Hawthorne Army Depot explosion. Grateful for their service.'

Nevada Gov Brian Sandoval said in a
statement: 'I am deeply saddened to hear of the incident at the
Hawthorne Army Depot this morning.

'The
men and women who work and train there put service ahead of self each
and every day. [My wife] Kathleen and I wish to extend our deepest
sympathies to those killed and their families. Our thoughts and prayers
are with those who have been injured and we pray for their speedy
recovery.'

Sen Kay Hagan, D-N.C., said in a statement: 'I was so saddened to learn about the seven Marines from Camp Lejeune who were killed last night in Nevada.

'My thoughts and prayers are with the
families of the Marines who were killed and those who were injured, and
I will continue to monitor the investigation so we can find out what
happened and take appropriate steps.'

Training: In a photo provided by the U.S. Army, military members practice during an evacuation training exercise at the Hawthorne Army Depot in 2008

It is one of the deadliest incidents to occur within a military facility in the U.S.

The
accident comes a week after a U.S. military plane assigned to a
Washington state Naval Air Station crashed during a routine training
flight, killing all three crew members on board.

In February 2012, seven Marines were killed when a chopper went two military choppers collided in the Chocolate Mountains along the California-Arizona border.

Thirteen service members were killed in the 2009 Fort Hood shooting, when Army psychiatrist Nidal Malik Hasan opened fire on colleagues at the Texas base.

In March 2002, two training accidents over the course of few days totaled five deaths.

In the first incident, a Navy HH-1 Huey chopper crashed into Split Mountain in the Sequoia National Forest. Two service members aboard the helicopter were killed and another four were injured.

A mortar explosion was reported a day later at Fort Irwin in the Mojave Desert. Three soldiers from Kansas were killed in that accident.